There a two ways of sending pictures within an e-mail:

1) The one you usually use with desktop e-mail clients (like Outlook), which consists in embedding pictures in every e-mail sent, encoded as Base64.

2) The way every e-mail marketing software (like MAILCast) in the market uses, that is, leaving pictures on the server for later downloading.

Why every e-mail marketing provider in the market chose the latter option?

There are several reasons for selecting this option, but mainly these important ones:

• If you embed pictures in every e-mail you send, they will be much bigger in size, will consume a lot of bandwidth, and (worst of all) maybe some recipients will not receive it due to size constraints. It is even possible that you'll annoy some of your recipients for sending them big mails that maybe are not of their interest.

• You have more chances of being considered spam by some spam filters because all these images are, in fact, sent as attachments (and probably you will generate a lot of attachments), and this could trigger the filters.

• Some e-mail clients don't have support for embedded images, most of them webmail.

• A huge amount of people catch their email on the move, whether it’s a blackberry, Windows Mobile or one of the more recent propietary handsets. HTML emails with embedded images are not suitable for this kind of devices because they take much longer to receive, and on some mobile networks that charge for data transfer costs a lot.

• Very importante one: If you don't leave images on the server is not possible to do tracking of the e-mail

As a personal thought I think it’s not a good idea to embed images in emails for the simple reason that it reduces the chances of your email getting through, which is the whole point of sending an email in the first place.